The invention essentially relates to a contact lens having at least one aspherical, progressive multifocal face, a process of preparation thereof and the use thereof as an intra-ocular implant to be substituted for the eye crystalline lens.
There is now an increased need for contact lenses allowing the vision both for far and near, for persons having difficulties for accommodation, notably the presbytic patients.
A number of solutions have been proposed these last years to solve this technical problem. Most of them are based on the location of a vision correction for near, at the periphery of the lens, different from the vision correction for far, by a simple movement of the lens at the time of lowering the eyes, in the path from the far vision (horizontal looking) to the near vision (see FR-A-No. 1 423 908 YSOPTIC).
In this movement, the lens, by bearing on the edge of the lower eye lid, is displaced upwards during the lowering of the looking and is putting in place the near correction before the pupil.
Two types of lenses have been proposed, the first type consisting in the performing of a near vision located at the periphery of the lens, in a restricted area of the lens, the rest of the lens performing a correction for far.
The second type of lens is generally conceived to perform a correction for near and the central portion is flattened to perform a correction for far. This lens is called a bifocal type lens. Thus, the vision for near is corrected at the periphery of the lens, the center performing a correction for far.
In the case where the lens is displaced by bearing on the edge of the lower eye lid, the lens does not provide a good correction. Indeed, in most cases, in view of the great number of parameters to be satisfied, the two corrections are subsisting and the subject has a double vision. The problem can only be solved in that case from 1 to 2% of the cases. These parameters are for instance the diameter of the restricted area, the position of the latter, the clearance of the lower eye lid, the diameter of the pupil, the importance of the displacement of the lens etc. . . .
A good working of the lens supposes the coexistence of two contradictory conditions, namely:
A sufficiently ample displacement of the lens, and
a good tolerance in the wearing of the lens which supposes a limitation of the displacements thereof.
On the other hand, with these bifocal or multifocal lenses, an additional problem lies in that at the border line of the portion correcting the vision for near with the portion for correcting the vision for far, a junction is obtained also called an intermediary line or a transition zone, due to the differences of curvature radius to obtain the bifocal or multifocal character of the lens. Such junctions are generating a junction or intermediary zone causing a jump in image. In the case of multifocal lenses, there is thus obtained a multiplication of the junctions, which is unacceptable.
To remedy to these drawbacks, other solutions have been proposed trying to introduce a progressive curvature or bend on the external or inner face of the lens to allow a progressive passage from the vision for far to the vision for near, either by the movement of the lens, or by performing a progressive power from the centre up to the edge of the lens, inside the pupil (simultaneous vision).
However, such solutions are again unsatisfactory as regards the quality of the correction, in particular with respect to the value of the near correction to be reached, as concerns the clearness of the corrected image.